Purifications of a serum factor (SBF) which can block specific cell-mediated serum immunity has been done by using immunoadsorbent columns. They have demonstrated that a SBF with a molecular weight of approximately 56,000 is both necessary and sufficient for the blocking activity. This factor has been shown to be glycoprotein. By performing experiments on two methylcholanthrene induced murine sarcomas with different tumor specific transplantation antigens, the 56,000 dalton factor could be shown to be specific with respect to its ability to block against tumor borne from the serum donor. Experiments are presently being performed trying to develop a radio-immunoassay for SBF on the basis of this information. We have also studied the in vitro sensitization of lymphocytes to tumor specific transplantation antigens. Lymphocytes from mice whose tumors have been excised could be sensitized for 5-8 days in vitro to cells of the same tumor and were then more capable of reacting to target cells carrying the respective tumor antigens than after sensitization only in vivo, demonstrating cytotoxicity at ratios as low as 1 lymphocyte per target cell in vitro. Lymphocytes from mice carrying tumors were initially cytotoxic but lost reactivity following 4 to 5 days in vitro contact with cells from the respective tumor. Lymphocytes from untreated control mice could be sensitized upon the in vitro exposure to antigens from approximately 15 days old mouse embryos. In collaboration with Dr. M. Bean we are studying the reactivity of melanoma patients to various cell lines derived from melanoma and from other tumors as well as to normal skin fibroblasts. Investigations are being made as to the nature of effector cells seen in this system, both for effector cell populations which are selectively reactive against melanoma cell lines and such which are nonselectively reactive against many cell lines. The evidence indicates that the selectively melanoma reactive cell is a T lymphocyte.